**1. Introduction**

Shallow geothermal energy resource is a kind of renewable resources, mainly referring to the low-temperature heat energy, also known as geothermal energy, in the earth's shallow surface within hundreds of meters (less than 200 m). Different from the traditional deep geothermal energy, it refers to the stratum thermal energy in the general layer, the temperature of which is less than 25°C, and its energy mainly comes from solar radiation and earth gradient warming. Compared with deep geothermal energy in the traditional sense, shallow geothermal energy is not affected by geological factors and generally exists in the surface of the earth, with many advantages such as wide distribution and renewable [1–3].

Due to the low temperature of shallow geothermal energy, which is not easy to be extracted, it was not used by people in the early years. With the progress of science and technology and the attention paid to the impact of natural environment, as a renewable, clean, and huge kind of energy, it has gradually been widely paid attention to. At present, the shallow geothermal energy is mainly used in the air conditioning system of buildings. The method is to extract the low-grade underground shallow

#### **Figure 1.**

*Schematic of ground source heat pump system.*

geothermal energy through the GSHP system and make use of it. **Figure 1** shows the schematic diagram of the GSHP system. By burying the GHE underground, it makes heat exchange with the relatively constant temperature stratum. In winter, it takes the earth as the high-temperature potential heat source and uses a small amount of highgrade energy (electric energy) to extract heat from the earth and provides heat for the user. On the contrary, in summer, it takes the earth as the low-temperature potential heat source and releases the excess heat of the user into the earth [4–8].

Traditional GSHP system can be divided into two kinds: water source heat pump technology and soil source heat pump technology. The heat source of the water source heat pump system is the underground or surface water, and the vast majority of the water source heat pump system directly extracts underground or surface water, which is an open system. The soil source heat pump system is a closed system by burying the GHE in the ground and exchanging heat with the surrounding rock and soil through the flow of the working medium inside the GHE. Because the underground or surface water used by water source heat pump system contains microorganisms, calcium and magnesium ions, and sulfate ions, the scale and blockage always occur in the system, resulting in poor system stability. At the same time, it is also restricted by hydrogeological conditions; thus, for the sake of realizing the development and utilization of shallow geothermal energy resources, the soil source heat pump system is widely used in the project at present.

#### *CFD Applications in Ground Source Heat Pump System DOI: http://dx.doi.org/10.5772/intechopen.109574*

Buried GHE is the most core component in the GSHP system. As shown in **Figure 2a**, the application research of the traditional U-type GHE started earlier and developed more mature. Therefore, the U type GHE is the most widely used, and most GSHP projects adopt this type of buried pipe. Compared with the U type GHE, the HGHE is a new type of geothermal exchanger, as shown in **Figure 2b**. Because the spiral pipe is arranged along the cylindrical wall, its heat transfer area and heat transfer amount are large, with low initial investment, high heat transfer efficiency, and other significant advantages, and HGHE with its unique advantages has been rapidly developed in recent years, effectively solving the economic and technical problems of traditional U-type GHE.

Due to the complicated hydrogeological conditions involved in the actual engineering of GHE, it is difficult to obtain the heat transfer process of buried pipe by experimental means due to the limitation of the region and testing conditions. For this reason, scholars around the world have carried out a lot of numerical heat transfer simulation works on GHE, mainly by using CFD technology to achieve the simulation research on the heat transfer characteristics and heat transfer process of buried pipes, for the sake of maximizing the heat transfer efficiency and improving the overall efficiency of the GSHP system. Angelo Zarrella [9–11] et al. established a numerical heat transfer model based on U-type and helix GHEs by using the electrical resistance analysis method. They considered the influence of external environment on soil boundary conditions and conducted experimental research. The model mainly considers the convective heat transfer of helical fluid, and the radial and axial heat conduction of soil and pile foundation, and ignores the heat transfer along the Angle direction. Salsuwanda Selamat [12] aiming at the design optimization problem of horizontal HGHE, established a three-dimensional numerical model and used CFD software for simulation calculation to simulate and optimize each parameter. Gyu-Hyun Go et al. [13] established a three-dimensional numerical model of horizontal HGHE and simulated its heat transfer characteristics. The accuracy of the model was verified through indoor thermal response experiment. Qiang Zhao et al. [14] established a threedimensional unsteady heat transfer model reflecting the heat transfer process of fluid and soil in the tube for the GHE and used the finite element method to solve and calculate. Then, the heat transfer characteristics of U-type, W-type, and helix GHEs are simulated and calculated, respectively. The research shows that the heat transfer efficiency of HGHE is higher than that of other two types of heat exchangers.

In order to further elaborate and introduce the application of CFD technology in the GSHP system, this chapter takes the new-type HGHE as an example and establishes its three-dimensional numerical model, which fully considers the external dynamic environmental conditions and initial soil temperature conditions. Then, the ANSYS\_FLUENT software, which is commonly used in CFD technology, is used to simulate and study the heat transfer characteristics of HGHE under different inlet water temperature conditions. Finally, the accuracy of the numerical model is verified by experimental means.
